Friday, 6 June 2014

Multi-band squid-pole dipoles and Yagi antenna: Feasibility?

Multi-band squid-pole dipoles and Yagi antenna: Feasibility?

One source of thin fibreglass for use with the loading coils on the TET-Emtron antenna I have been working on is telescopic "squid" poles (used for catching squid). A 7m squid pole is in seven sections, a metre long, from 45mm to 8mm for the heavy-duty version. The poles only cost about $30 each. They are available from http://haverford.com.au/telescopic-poles.html.

Given that I was thinking of the TET Yagi elements using the tricks of vertical antenna to reduce length, I thought, why not go the other way and use telescopic squid-poles as rotatable dipoles or Yagi elements?

Telescopic elements are quite useful for me as my tilting mast tilts to the short end of a trapezoid, with only 3 to 4m for a boom, compared to 10m at the other end. With the mast right down, I can mount the boom and one side of a Yagi horizontally, rotate the boom 90 degrees to its correct orientation, then extend the elements on the other side of the boom as I raise the mast.

Squid-poles and wire dipoles or Yagi antenna have the advantage of being easy to dismantle, and could be used as a portable antenna.

One of the reasons driving this project that small multi-band beams are either not available or too expensive. TET-Emtron are currently re-designing their antenna, so there may be an option in the future.

Mono-band, full-size dipole or Yagi

The squid poles are long enough, at 7m for a full-size 20m dipole. Just mount two squid poles on a piece of rectangular aluminium and bolt it to the tube at the top of a mast. The antenna can be a piece of wire, or aluminium strips, held on with zip-ties and driven from the centre.

I tried coiling the wire around the poles, but that seemed to create an inductor and increase the SWR. Straight lengths of wire seemed to work best.

As I have a narrow yard, a two element Yagi with close-coupling and both elements driven, as per the TET design seems the best. So for a $100 or so, a 2 element, 20m Yagi seems possible. My local aluminium fabricator sells 50mm x 3mm tube for $15/m for use as a boom. The clamps are pretty much the more expensive bits.

Multi-band, full-size dipole or Yagi

Given that the squid pole is an insulator, it is possible to attach a number of different lengths of wire (or aluminium strip), cut to different wavelength. All conductors on each side are joined at the centre-end and can be driven, preferably with a balun or similar. The arrangement is not too dis-similar to a multi-band wire antenna such as those made by Alpha Delta http://www.alphadeltacom.com/pg1.html, but are rotatable.

The conductors can be attached around the pole, or even extended down from it with spacers, like the Alpha-Delta wire antenna. It could be possible to suspend the Alpha-Delta wire antenna from the poles.

Again, two or more elements could be used to make a low-cost, light-weight Yagi.

Close-coupled, dual-drive radiator and reflector

A close-coupled, dual-drive radiator and reflector is used to increase the front to back ratio. Such a system is used in the pre-1985 TET antenna I have and in the Ca-Av Labs beams http://www.cal-av.com/antennas.html. There is a PDF description of the drive system on their page. The drive is based on prior technology and a number of current and expired patents. The main patent seems to rely on using the lower velocity-factor of coaxial cable to make the boom length and feed system proportionally shorter.

Does it work?

I zip-tied a couple of squid poles to a piece of timber. I then used the insulated conductors from electrical wire, at three random lengths, strapped to the pole with gaffer tape. With the crude dipole sitting on a big plastic rubbish bin, I used my Rig-Expert AA-600 antenna analyser to see what was happening: three points of resonance, as expected.






Note the range on the analyser. In the first photo it is +/- 187 kHz, but in the second is +/- 6 MHz, thus showing resonance for the 3 pieces of wire. The lower frequency is probably more affected by the ground.

The RigExpert antenna analyzers are very good; the last photo is very impressive. I may do a post on it, once I have a better understanding of how to use it.

Shortened multi-band dipole or Yagi

While a compromise, it should be possible to use loading coils to effectively shorten the element length. Fully extended, the squid poles are 7m long but only 8mm at the tip. Not using the last two sections reduces the length to 5m, but gives about 20mm at the tip.

While traps and loading coils could be used with a single conductor, it is probably easier to use multiple conductors and loading coils on the conductors needing to be longer than 5m, such as for 20 and 40m. With loading coils, it may be better to suspend some conductors below the pole, per Alpha Delta arrangement.

Capacitive hats, per TET, could be used to electrically shorten the elements, but they may be too heavy or difficult to mount.

Where to next?

I have two potential ways of making a small 2 element 40/20/15/10 Yagi, but only need one. Probably out of curiosity I will build both and try them. Tuning the antenna, especially with the close-coupling, dual drive most likely will be problematic. One immediate problem is how long to make the boom, more specifically to which band? A 1/10 wavelength for 40m is 4m, which should fit my yard, just... I probably want best performance on 20m.  Neither will be particularly robust, for storms or birds, but I don't mind too much.

To be continued...



1 comment:

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